The current revolution in wireless communications and the need for smaller wireless communications devices has spawned significant efforts directed to the optimization and miniaturization of radio communications electronic devices. Passive components (such as inductors, capacitors and transformers), play a necessary role in the operation of these devices and thus efforts are directed toward reducing the size and improving the performance and fabrication efficiency of such passive components.
Discrete inductors and capacitors are electromagnetic components employed in alternating current and radio frequency applications, such as oscillators, amplifiers and signal filters, to provide frequency dependent effects. Specifically, the voltage across the inductor is a function of the product of the inductance and the time derivative of the current through the inductor. A conventional inductor comprises a plurality of windings enclosing a core constructed of a ferromagnetic or an insulating material. Although an inductor core is not required, use of a ferromagnetic core, for example, increases the inductance value. The inductance is also a function of the number of coil turns (specifically, the inductance is proportional to the square of the number of turns) and the core area. Conventional discrete inductors are formed as a helix (also referred to as a solenoidal shape) or a torroid. The core is typically formed of iron, cobalt or nickel (or a ferromagnetic alloy) comprising a plurality of magnetic domains. The current supplied to the inductor induces a magnetic field in the core material, causing domain alignment and a resulting increase in the material permeability, which in turn increases the inductance.
Developments in the semiconductor industry have over the years been directed at fabricating higher performance devices of decreasing size. One challenge of semiconductor circuit design and fabrication is the integration of high performance capacitors and inductors into the semiconductor device. Ideally, these components are formed on a relatively small surface area of a semiconductor substrate, using methods and procedures that are conventional in the semiconductor fabrication art. However, compared with the feature sizes and line widths of active devices, inductors and capacitors are large and not easily integrated into semiconductor devices that typically have feature sizes in the sub-micron range.
Most inductors formed on a semiconductor substrate surface have a spiral shape, where the plane of the spiral is parallel to the substrate surface. Many techniques are known for forming the spiral inductor, such as by masking, patterning and etching a layer of conductive material formed on the substrate surface. Multiple interconnected spiral inductors can also be formed to provide the desired inductive properties and/or to simplify the fabrication process. See for example, U.S. Pat. No. 6,429,504 describing a multi-layer spiral inductor and U.S. Pat. No. 5,610,433 disclosing a high value inductor with a high Q factor formed from a plurality of layers with each layer comprising two or more coils. The coils in the various layers are interconnected in series.
The Q (or quality factor), an important inductor figure of merit, is defined as the ratio of inductive reactance to resistance. High-Q inductors (e.g., having a low resistance) present a narrow Q peak as a function of the input signal frequency, where the peak occurs at the inductor resonant frequency. High-Q inductors are especially important for use in frequency-dependent circuits operating with narrow bandwidths. For example, increasing the Q for an inductor operating in an oscillator decreases the oscillator phase noise, and confines the oscillator frequency to a narrower band of frequencies. Because the Q value is an inverse function of inductor resistance, minimizing the resistance increases the Q. One known technique for minimizing the resistance increases the cross-sectional area of the conductive material forming the inductor. However, such conductors can present difficulties during the etching, cleaning, and passivating steps, and consume valuable space on the semiconductor substrate. Also, difficulties arise when gaps form in the dielectric material between the metal conductors, creating device reliability problems, as the gaps can promote local dielectric breakdown and consequent shorting of the inductor turns.
The magnetic field of a spiral inductor formed on the surface of a semiconductor substrate is perpendicular to the substrate. The field induces eddy currents within the semiconductor substrate and the conductive interconnect structures formed in the substrate to connect active semiconductor regions. Since these eddy currents represent a loss mechanism, they increase the inductor resistance and thus lower the inductor Q factor. To avoid such losses, at relatively low operational frequencies the inductive effect can be achieved by simulating an inductor with active devices. But active devices have a finite dynamic range, can inject unwanted noise into the operating circuits and cannot provide acceptable inductive effects at higher operational frequencies.
One known technique for limiting eddy current losses places a conductive shield parallel to and underlying the inductor. The conductor shorts the magnetic field lines, reducing the eddy currents in the underlying semiconductor substrate material. Eddy currents can also be reduced by using deep trench technology to remove a region of silicon below the inductor. This technique, which can be used to form isolation regions in an integrated circuit comprising both bipolar and CMOS transistors (complementary metal-oxide silicon field effect transistors), removes the silicon from the front or top surface of the wafer. Increasing the distance between the inductor and the underlying silicon also lowers the eddy current losses. Marginal improvement in the Q factor have been noted when these techniques are employed.